The present invention relates to an image forming method in which recording is carried out by ejecting ink from an ink-jet recording head and an image forming apparatus, and specifically to an ink-jet recording method as well as an ink-jet recording apparatus which is driven at a high rate.
In ink-jet recording, there are a system in which images are formed by continuously ejecting ink and varying the ejected direction of only ink droplets necessary for image formation, while utilizing an electrode arranged between the image recording body (the medium) and the ink head, and an on-demand type recording system in which ink droplets are ejected only as needed.
From the viewpoint of operating cost as well as simplicity of the unit, said on-demand type is more advantageous. Methods for applying pressure to ink in an ink chamber in the case of on-demand include a method in which pressure is applied to ink utilizing deformation of an element (being a piezo element) in the ink chamber, which is deformed when voltage is applied, and a method in which an electric current is applied to a heat-generating resistor so that ink volume is increased due to vaporization of ink components.
In recent years, the market has required high speed for ink-jet printing. Specifically, for the on-demand type, it has been demanded that printing be carried out by ejecting water-based ink at a high rate.
In order to achieve high speed printing, it is necessary that ink ejection frequency per unit time be increased by applying voltage to the electric actuator, such as an electrode or heat-generating resistor, at a high frequency. In order to meet market demand, driving frequency is preferably at least 15 kHz, is more preferably at least 20 kHz, and is further more preferably at least 30 kHz.
However, when the head is driven at such a high rate, the ejected ink rate tends to decrease during continuous ejection. When the ejected rate decreases, it is impossible to form highly detailed images due to an increase of fluctuation in the size of ink spreading as well as in the ink droplet-adhered position. Other than these, problems also occur in which the working life of the head is shortened due to the fact that the electrode or the electric circuit is damaged due to the high rate driving.
In an ink-jet printer in which ink is ejected while compressing the ink chamber utilizing a piezo element, it is effective that in order to efficiently use the ejection energy, said piezo element, in which the electrode is arranged, closely approaches said ink. When said piezo element is arranged to closely approach said ink, the electrode, which drives said piezo element, closely approaches said ink.
However, when the electrode comes into direct contact with the ink, the electrode, when voltage is applied, tends to erode. In order to minimize said erosion, an insulating layer can be provided between the electrode and the ink.
An increase in the thickness of said insulating layer hinders piezo motion resulting in energy loss.
On the other hand, there is an ink-jet recording apparatus having such a structure that by generating heat energy while applying voltage to a heat-generating resistor, air bubbles are generated in said ink so that ink is ejected. In said apparatus, also, in order to minimize erosion of the heat-generating resistor as well as the electrode, the heat-generating resistor and the electrode are covered with an insulating layer.
However, the use of the thin layer causes problems in which the working life of the electrode is shortened.
Accordingly, the market has increasingly been demanding that even in high speed recording, ejection rate be stabilized, heads exhibit sufficiently long working life, and running cost be not increased.
Japanese Patent Publication Open to Public Inspection No. 8-20738 describes that by adjusting the electric conductivity of ink to 1.0 mS/cm or less, electrode erosion is minimized and more stable printing is achieved.
However, when in order to realize high speed printing, the head is driven at a high rate, electrode degradation occurs even though the electrical conductivity is adjusted to said optimal range.
Japanese Patent Publication Open to Public Inspection No. 11-209670 describes that by employing an ink-jet printer in which the total dissolved gas concentration is adjusted to be 2,950 ppb or less, lack of ejected ink can be minimized.
However, said patent publication does not describe any head which is driven at a high rate.
An object of the present invention is to provide an ink-jet recording method, an ink-jet recording apparatus, and an ink-jet head capable of recording stable images at a high rate over an extended period of time.
The above objects of the present invention can be achieved by following structures.
[Structure 1]
An ink-jet recording method of forming an image with an ink-jet head, wherein the ink-jet head comprises an ink chamber, an electric actuator provided in the ink chamber and an insulating layer covering the electric actuator, comprising steps of:
feeding an ink containing a coloring material and a water-soluble solvent into ink chamber, and
applying a driving voltage with a driving frequency of 10 kHz to 55 kHz onto the electric actuator so that the ink is jetted from the ink chamber so as to form the image;
wherein the thickness of the insulating layer is 0.1 xcexcm to 10 xcexcm, and the concentration of oxygen dissolved in the ink is 4 ppm or less.
[Structure 2]
The ink-jet recording method of Structure 1, wherein the electric actuator is an electrode.
[Structure 3]
The ink-jet recording method of Structure 2, wherein the ink chamber comprises a piezo element and the electrode is provided on the piezo element.
[Structure 4]
The ink-jet recording method of Structure 1, wherein the electric actuator is a heat-generating resistor.
[Structure 5]
The ink-jet recording method of Structure 1, wherein pH of said ink is 7 or more.
[Structure 6]
The ink-jet recording method of Structure 1, wherein said coloring material is a pigment.
[Structure 7]
The ink-jet recording method of Structure 1, wherein the sum of a concentration of sulfate ion, chloride ion and nitrate ion in said ink is 500 ppm or less.
[Structure 8]
The ink-jet recording method of Structure 1, wherein the sum of a concentration of sodium ion and potassium ion in said ink is 500 ppm or less.
[Structure 9]
The ink-jet recording method of Structure 1, wherein said driving frequency is 20 kHz or more.
[Structure 10]
The ink-jet recording method of Structure 9, wherein said driving frequency is 30 kHz or more.
[Structure 11]
The ink-jet recording method of Structure 1, wherein a surface tension of said ink is from 31 to 39 mN/m.
[Structure 12]
The ink-jet recording method of Structure 1, wherein the concentration of the dissolved oxygen in said ink is 2 ppm or less.
[Structure 13]
The ink-jet recording method of Structure 1, wherein the concentration of the dissolved oxygen in said ink is 0.01 ppm or more.
[Structure 14]
The ink-jet recording method of Structure 1, wherein the thickness of said insulating layer is from 0.1 to 5 xcexcm.
[Structure 15]
An ink-jet recording apparatus for forming an image, comprising:
an ink accommodating section having an ink chamber, an electric actuator provided in the ink chamber, and an insulating layer covering the electric actuator, wherein the thickness of the insulating layer is 0.1 xcexcm to 10 xcexcm;
a driving section to apply a driving voltage onto the electric actuator with a driving frequency of 10 kHz to 55 kHz so that an ink is jetted from the ink chamber so as to form the image, and
an ink feeding section to feed the ink into the ink chamber, wherein the ink contains a coloring material and a water-soluble solvent and the concentration of oxygen dissolved in the ink is 4 ppm or less.
[Structure 16]
The ink-jet recording apparatus of Structure 15, wherein the electric actuator is an electrode.
[Structure 17]
The ink-jet recording method of Structure 16, wherein the ink chamber comprises a piezo element and the electrode is provided on the piezo element.
[Structure 18]
The ink-jet recording apparatus of Structure 15, wherein the electric actuator is a heat-generating resistor.
[Structure 19]
The ink-jet recording apparatus of Structure 15, wherein pH of said ink is 7 or more.
[Structure 20]
The ink-jet recording apparatus of Structure 15, wherein said coloring material is a pigment.
[Structure 21]
The ink-jet recording apparatus of Structure 15, wherein the sum of a concentration of sulfate ion, chloride ion and nitrate ion in said ink is 500 ppm or less.
[Structure 22]
The ink-jet recording apparatus of Structure 15, wherein the sum of a concentration of sodium ion and potassium ion in said ink is 500 ppm or less.
[Structure 23]
The ink-jet recording apparatus of Structure 15, wherein said driving frequency is 20 kHz or more.
[Structure 24]
The ink-jet recording apparatus of Structure 23, wherein said driving frequency is 30 kHz or more.
[Structure 25]
The ink-jet recording method of Structure 15, wherein a surface tension of said ink is from 31 to 39 mN/m.
[Structure 26]
The ink-jet recording apparatus of Structure 15, wherein the concentration of the dissolved oxygen in said ink is 2 ppm or less.
[Structure 27]
The ink-jet recording apparatus of Structure 15, wherein the concentration of the dissolved oxygen of said ink is 0.01 ppm or more.
[Structure 28]
The ink-jet recording apparatus of Structure 15, wherein the thickness of said insulating layer is from 0.1 to 5 xcexcm.